Recent studies have suggested that the hypothalamus may play a central role in the development and maintenance of certain forms of experimental hypertension. Plasma from volume-expanded hypertensive animals and patients with essential hypertension has been reported to contain a substance which inhibits (Na,K)ATPase. Sodium pump suppression in vascular smooth muscle caused by such a circulating factor could result in tonic increases in vascular tone, increased vascular sensitivity to vasoactive agents, and arterial hypertension. In certain of the animal models of hypertension it appears that an intact hypothalamus is required for the expression of the sodium-pump suppressing activity. Bovine hypothalamus contains a acid-stable low molecular weight substance which is a potent, reversible inhibitor of the mammalian (Na,K)ATPase, and possesses the characteristics of the putative natriuretic hormone. This factor may be a chemical mediator linking kidney, brain, and the cardiovascular system in the genesis of experimental volume-expanded and certain forms of human essential hypertension. The aims of this proposal are: (1) To obtain reliable assays of the inhibitory factor based on Mg2+ dependent reversible inhibition of purified (Na,K)ATPase, and inhibition of ouabain-sensitive 86Rb+ uptake in human erythrocytes; (2) To use the assays to facilitate purification of the inhibitor by high performance liquid chromatography, affinity chromatography, and chemical manipulation of the molecule; and (3) To determine the chemical nature of the purified inhibitor. Success in the project would make possible direct physiological testing to confirm the inhibitor's role in hypertensive disease as well as in other disorders of altered sodium metabolism and membrane transport phenomena.